Endodontic therapy is performed to save a tooth when the pulp, the soft tissue in the center of the tooth, becomes infected or damaged. The therapy includes opening the tooth, removing the pulp, cleaning, shaping, and smoothing the dentinal walls, and then filling the tooth. It is important, and in fact necessary, that the pulp be removed not only from the pulp chamber in the crown of the tooth but also from the root canals which extend to the apices (at the root end) of the tooth. Failure to completely remove the pulp can render the entire procedure ineffective, leading to loss of the tooth.
A wide variety of instruments, such as files, reamers, rasps, broaches, and probes, have been developed over the years for use in removing the pulp and cleaning, shaping, and smoothing the walls of the root canal. One example is the K-type file, which is formed from a blank which usually has a square cross section. By tightly twisting the blank, a file having continuous spiral cutting edges is formed. A K-type reamer is formed in much the same fashion, although usually with a looser twist, and usually from a blank with a triangular cross section. A K-flex file is similar to the standard K-type file, except it is formed by twisting a blank with a rhomboid cross section. Another example is the H-type (Hedstrom) file, which is made by machining a cylindrical blank to form a helical cutting edge. All of the instruments described above become successively larger from the top toward the handle, usually for a length of 16 mm.
Many of these instruments are designed to be manipulated by hand, typically having a handle at their distal end. Others are adapted to be held in a mechanical hand piece. Hand manipulated root canal instruments achieve their cutting and cleaning objectives by being rotated, or by being moved in and out of the root canal along the instrument's longitudinal axis. Mechanically manipulated instruments achieve their cutting and cleaning objectives by being vibrated from side to side, rotated, moved in and out longitudinally, or by a combination of longitudinal and rotational movement.
When cleaning and shaping a root canal, the desired goal is to produce a funnel-shaped canal, with the smaller end at the apical foramin (hole). This funnel shape makes it easier to fill the root canal. At the same time, great efforts are made not to remove too much dentin, because that can weaken the structural integrity of the tooth.
Even with the wide diversity of available instruments, there are some common problems in endodontic therapy for which no existing instrument provides a very satisfactory solution. For example, the multiple root canals in molars are usually curved, in addition to having an extremely small canal diameter which becomes even smaller at their apices. The curvature is three dimensional, and frequently is irregular from point to point in a given root canal. This combination of curvature and small diameter makes it extremely difficult to ensure complete pulp removal and satisfactory cleaning, shaping, and smoothing of the dentinal walls without removing excessive dentin and thus changing the curvature of the canal and weakening the root structure. Various embodiments of this undesired result are referred to as transportation, ledging, and zipping. Conservative therapy using instruments which have smaller diameters can help avoid ledging, transportation, and zipping, but tends to leave too much pulp in the root canal, which is very undesirable.
This difficulty is compounded by the length of the cutting surface on most endodontic instruments. Most such instruments are designed to cut along most of their length, and this makes the undesired results listed above even harder to avoid.
There is a long-standing need for an instrument which can clean and shape curved root canals while maintaining as much as possible the original configuration of the canal.